The present invention is directed to the field of antennas used for RF data communications devices, particularly those used to transmit and receive digital signals, e.g. two-way pagers and the like. There has been a proliferation in recent years in the field of RF telecommunications with items such as cordless and cellular telephones becoming commonplace items. Pagers, in particular, have become common among individuals who need to be quickly contacted from remote locations, e.g. technicians, etc. With such devices, it is very important to maintain a clear, strong signal that preserves the integrity of the data transmission.
The antennas used with previous RF data communication devices are prone to many significant problems. Some devices, such as pagers are usually worn on the person of the user. However, the human body has certain inherent dielectric properties (e.g. due to charge and current fluctuations, etc.) that create an electromagnetic boundary. The inherent boundary conditions of the body of the user changes the surrounding impedance, affecting the antenna current distribution and the signal radiation pattern, thus lowering the gain of the antenna by about 4 dB. In this way, the antenna is "detuned." Antenna detuning is also caused by the presence of certain objects (e.g. metallic bodies) and also various ground plane conditions. This effect results in a shorter operating radius and poor in-building performance for RF data communications devices, especially pagers.
Previous devices also suffer from performance problems related to the polarization characteristics of the transmission and reception signals. Electromagnetic radiation propagates in any plane and can thus be regarded as having vertical and horizontal polarizations. In order to receive a strong signal, an antenna must be properly aligned with the polarization plane of the incoming signal. However, when a device is in operation, it may be turned in all different directions and may not be optimally aligned to receive an incoming signal. In a two-way device, a similar problem results in transmission from the device. Previous device antennas incorporate a loop design, which is nominally effective at implementing the two polarizations but suffers from low gain and low bandwidth. Environmental sources also affect the reception of a polarized signal. For example, the metal in buildings effectively "tips" a vertically polarized wave, thus weakening the strength of a signal received with a vertically polarized antenna.
One method of addressing the above-noted limitations imposed by signal reception in an RF data communications device, such as a pager, is to establish two-way communication, so that an acknowledgment or reply signal is transmitted from the pager back to the source. However, because these devices are usually worn or used in close proximity to the user's body, the electromagnetic boundary around the user's body also sharply reduces transmission efficiency. Also, transmission bandwidths as low as 1/2% are typical with previous two-way pagers. In these ways, the antennas of previous RF data communications devices do not provide the reliable and efficient operation necessary for the transmission and reception of a digital signal.